2015ApJ...800...25T

We present the results of interferometric spectral line observations of Arp 220 at 3.5 mm and 1.2 mm from the Plateau de Bure Interferometer, imaging the two nuclear disks in H¹³CN(1-0) and (3-2), H¹³CO⁺(1-0) and (3-2), and HN¹³C(3-2) as well as SiO(2-1) and (6-5), HC¹⁵N(3-2), and SO(6₆-5₅). The gas traced by SiO(6-5) has a complex and extended kinematic signature including a prominent P Cygni profile, almost identical to previous observations of HCO⁺(3-2). Spatial offsets 0.''1 north and south of the continuum center in the emission and absorption of the SiO(6-5) P Cygni profile in the western nucleus (WN) imply a bipolar outflow, delineating the northern and southern edges of its disk and suggesting a disk radius of ∼40 pc, consistent with that found by ALMA observations of Arp 220. We address the blending of SiO(6-5) and H¹³CO⁺(3-2) by considering two limiting cases with regards to the H¹³CO⁺ emission throughout our analysis. Large velocity gradient modeling is used to constrain the physical conditions of the gas and to infer abundance ratios in the two nuclei. Our most conservative lower limit on the [H¹³CN]/[H¹³CO⁺] abundance ratio is 11 in the WN, compared with 0.10 in the eastern nucleus (EN). Comparing these ratios to the literature we argue on chemical grounds for an energetically significant active galactic nucleus in the WN driving either X-ray or shock chemistry, and a dominant starburst in the EN.